Part of the Hajj entails running back and forth seven times between the hills of Safa and Marwa. In the Quran, these hills are mentioned, but the practice is never elaborated on:
Surah 2:158
158 Indeed, ˹the hills of˺ Ṣafa and Marwah are among the symbols of Allah. So whoever performs the pilgrimage or minor pilgrimage, let them walk between ˹the two hills˺. And whoever does good willingly, Allah is truly Appreciative, All-Knowing.
Why in the world God wants us to walk between these two hills, or in what sense they are a "symbol" is never explained. Symbol is a word with a meaning. What exactly do these hills symbolize or represent? Nowhere in the Quran are we given an answer, even though the Quran frequently claims to be "fully explained" (Surah 6:114, 15:1, 16:89, 24:46, 26:2, 27:1, 28:2, 41:3, 57:9, etc.).
When we venture outside of the Quran into the Hadith, we find that this practice, like worship of the Black Stone, and circumambulating the Kaaba, is yet another relic of local Arabian Paganism:
Sahih al-Bukhari 3847
Narrated Ibn 'Abbas:
To run along the valley between two green pillars of Safa and Marwa (mountains) was not Sunna, but the people in the pre-islamic period of ignorance used to run along it, and used to say: "We do not cross this rain stream except running strongly."
Sahih al-Bukhari 4496
Narrated `Asim bin Sulaiman:
I asked Anas bin Malik about Safa and Marwa. Anas replied, "We used to consider (i.e. going around) them a custom of the Pre-islamic period of Ignorance, so when Islam came, we gave up going around them. Then Allah revealed: "Verily, Safa and Marwa (i.e. two mountains at Mecca) are among the Symbols of Allah. So it is not harmful of those who perform the Hajj of the House (of Allah) or perform the Umra to ambulate (Tawaf) between them." (2.158)
Similar narrations can be found in Sahih al-Bukhari 1643, 1648, and Jami at-Tirmidhi 2966.
And, in Sahih Muslim 1277a, we also see that another aspect of the Hajj - people cutting their hair or shaving their head - was also part of the pre-Islamic Pagan practice associated with Safa and Marwa:
Sahih Muslim 1277a
The Ansar in the Days of Ignorance pronounced the Talbiya for two idols, fixed on the bank of the river which were called Isaf and Na'ila. The people went there, and then circumambulated between al-Safa' and al-Marwa and then got their heads shaved. With the advent of Islam they (the Muslims) did not like to circumambulate between them as they used to do during the Days of Ignorance. It was on account of this that Allah. the Exalted and Majestic, revealed:" Verily al-Safe and al-Marwa are among the Signs of Allah" to the end of the verse. She said: Then people began to observe Sa'i.
Therefore, according to Islam's own sources, two aspects of Hajj - walking between two hills, and cutting one's hair - were simply elements of local Arabian Paganism that he absorbed into his new false religion.
The Hadith contain many narrations regarding Muhammad's behavior around these hills, for instance:
Sunan an-Nasai 2979
It was narrated that Ibn Abbas said:
"The Prophet walked rapidly between As-Safa and Al-Marwah to show the idolaters that he was strong."
Sunan Ibn Majah 2987
It was narratd that an Umm Walad of Shaibah said:
“I saw the Messenger of Allah (ﷺ) performing Sa’y between Safa and Marwah saying: ‘The valley should not be crossed except quickly.’”
Isn't it strange how what Muhammad was doing was the same thing that the Pagans used to do before Islam? Of course, if you asked Muhammad about it, he would assure you that Abraham, and Noah, and Moses, and Jesus all did this, with his trademark no evidence whatsoever to support anything he's saying.
Muhammad claimed prolifically to be continuing in the tradition of the Biblical prophets (Surah 3:3-4, 5:43-48, 5:68, etc.). However, his religion consists of practices which no Jew or Christian have ever been obliged to follow, but were instead followed by Arabian Pagans that were local to Muhammad - running 7 times between two hills in Muhammad's hometown, circumambulating a cubic Pagan shrine from Muhammad's hometown 7 times, praying towards a cubic Pagan shrine, and kissing a Black Stone in the corner of that cubic Pagan shrine, are some prominent examples of this.